That does young adults a disservice. If we can send them out to die at 18 in the military, to say you're not an adult for another 12 years is wrong.
It also denies agency to people who are marrying, having children, etc at that stage.
I'll agree that you're still finding yourself and learning in your twenties, but acting like 30 is the new 20 infantilizes young adults when the world NEEDS them to grow up.
Look at Social Justice Warriors screaming at a dean about saying don't freak out over Halloween costumes and use of the term "black hole" while ISIS has sex slaves, atheists are murdered by mobs in Pakistan and we have a whole host of economic and social problems to resolve.
The only people who benefit from saying "you 28 year old, shut up and do what the adults say" are the older people who want them to stay out of the political process and vote for said elders because they are kept dependent on parents or, more likely, the state.